linux-kernel - Re: [PATCH 08/22] gpiolib: cdev: complete the irq/thread timestamp handshake

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Date:   Thu, 25 Jun 2020 11:44:30 +0200
From:   Bartosz Golaszewski <bgolaszewski@...libre.com>
To:     Kent Gibson <warthog618@...il.com>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        linux-gpio <linux-gpio@...r.kernel.org>,
        Linus Walleij <linus.walleij@...aro.org>
Subject: Re: [PATCH 08/22] gpiolib: cdev: complete the irq/thread timestamp handshake

On Wed, Jun 24, 2020 at 4:08 PM Kent Gibson <warthog618@...il.com> wrote:
>
> On Wed, Jun 24, 2020 at 04:00:42PM +0200, Bartosz Golaszewski wrote:
> > wt., 23 cze 2020 o 06:02 Kent Gibson <warthog618@...il.com> napisał(a):
> > >
> > > Reset the timestamp field to 0 after using it in lineevent_irq_thread.
> > >
> > > The timestamp is set by lineevent_irq_handler and is tested by
> > > lineevent_irq_thread to determine if it is called from a nested theaded
> > > interrupt.
> > > lineevent_irq_thread is assuming that the nested, or otherwise, status
> > > of the IRQ is static, i.e. it is either always nested or never nested.
> > > This change removes that assumption, resetting the timestamp so it can
> > > be re-used to determine the nested state of subsequent interrupts.
> > >
> > > Signed-off-by: Kent Gibson <warthog618@...il.com>
> > >
> >
> > This change makes sense to me but I'm having a hard time processing
> > the explanation. If we're requesting the interrupt and allocating the
> > lineevent state in the same function - how can we run into a situation
> > here the status of the irq would change like what you describe?
> >
>
> I'm not totally sure myself, as my understanding of how interrupts are
> shared in the kernel is pretty sketchy, but my concern is that if we
> are sharing the irq then whoever we are sharing with may release the irq
> and we go from nested to unnested.  Or vice versa.  Not sure if that is
> valid, but that was my concern, and it seemed like a minor change to
> cover it just in case.
>

It's my understanding that a shared interrupt must be explicitly
requested as shared by all previous users or request_irq() will fail.
In this case: we call request_threaded_irq() without the IRQF_SHARED
flag so it's never a shared interrupt. Even if someone previously
requested it as shared - our call will simply fail.

I still think that resetting the timestamp is fine because it's not
being set to 0 in hardirq context. We just need a different
explanation.

Bart